Multi-criteria selection of structural adhesives to bond ABS parts obtained by rapid prototyping

One of the most used methods in rapid prototyping is Fused Deposition Modeling (FDM), which provides components with a reasonable strength in plastic materials such as ABS and has a low environmental impact. However, the FDM process exhibits low levels of surface finishing, difficulty in getting complex and/or small geometries and low consistency in “slim” elements of the parts. Furthermore, “cantilever” elements need large material structures to be supported. The solution of these deficiencies requires a comprehensive review of the three-dimensional part design to enhance advantages and performances of FDM and reduce their constraints. As a key feature of this redesign a novel method of construction by assembling parts with structural adhesive joints is proposed. These adhesive joints should be designed specifically to fit the plastic substrate and the FDM manufacturing technology. To achieve this, the most suitable structural adhesive selection is firstly required. Therefore, the present work analyzes five different families of adhesives (cyanoacrylate, polyurethane, epoxy, acrylic and silicone), and, by means of the application of technical multi-criteria decision analysis based on the analytic hierarchy process (AHP), to select the structural adhesive that better conjugates mechanical benefits and adaptation to the FDM manufacturing process.     

Analysis of mixed adhesive joints considering the compaction process

Incorporating a material properties variation along the bondlines has proved to be a useful method for improving adhesive joints performance. In this work, the potential of the technique is analysed for a single lap joint using the mixing adhesives approach. In order to include the compaction process effect in the structural analysis during the joint assembly, a computational fluid-dynamic model capable of integrating different resins along the bondline has been developed. Then, the results obtained from this model are mapped into a finite element model through an application developed for this purpose. Several parametric studies have been carried out comparing different configurations in terms of maximum load capacity of the joints. Finally, one of these joints configurations has been manufactured using a special device developed for assembling these mixed adhesive joints and tested. This banded configuration have shown both numerically and experimentally an ultimate load improvement of over 70%.     

Blistering of Glass-Epoxy Amine Adhesive Joints in Water Vapour at High Pressure. An Indication of Interfacial Crumpling

When joints are made between solids, the surfaces of which are soluble in water, with an epoxy-amine adhesive, blistering is observed in water vapour at high temperature and pressure. The occurrence of these blisters follows the cavitation theory in elastomers. Microscopic observation of the blisters suggest that they come from initial smaller ones which are homogeneously spread along the interface (not air bubbles or defects) and grow under the osmotic pressure developed by water condensation in the initial “sucker” and by surface dissolution. These observations led us to suggest that the solid/adhesive interface crumples during hardening of the adhesive and that many small suckers exist along the interface. This leads to a new model for the loss of adherence of epoxy-metal joints kept in high humidities.     

Three-parameter,elastic foundation model for analysis of adhesively bonded joints

A novel three-parameter, elastic foundation model is proposed in this study to analyze interface stresses of adhesively bonded joints. The classical two-parameter, elastic foundation model of adhesive joints models the adhesive layer as a layer of normal and a layer of shear springs. This model does not satisfy the zero-shear-stress boundary conditions at the free edges of the adhesive layer due to the inherent flaw of the two-parameter, elastic foundation model, which violates the equilibrium condition of the adhesive layer. To eliminate this flaw, this study models the adhesive layer as two normal spring layers interconnected by a shear layer. This new three-parameter, elastic foundation model allows the peel stresses along the two adherend/adhesive interfaces of the joint to be different, and therefore, satisfies the equilibrium condition of the adhesive layer. This model regains the missing “degree of freedom” in the two-parameter, elastic foundation model of the adhesive layer by introducing the transverse displacement of the adhesive layer as a new independent parameter. Explicit closed-form expressions of interface stresses and beam forces are obtained. The new model not only satisfies all boundary conditions, but also predicts correctly which interface has the strongest stress concentration. The new model is verified by continuum models existing in the literature and finite element analysis. The new three-parameter, elastic foundation model provides an effective and efficient tool for analysis and design of general adhesive joints.     

Influence of the mechanical behaviour of different adhesives on an interference-fit cylindrical joint

Hybrid adhesive joining techniques are often used in many industrial sectors to design lightweight structures. A hybrid adhesive joint results from the combination of adhesive bonding with other traditional joining methods such as welding and mechanical fastening, with the aim of combining the advantages of the different techniques and overcoming their drawbacks.This study focuses on the interference fitted/adhesive bonded joining technique. In this application, two cylindrical components are coupled together by inserting one into the other, after having placed an adhesive between them. Generally anaerobic acrylic adhesives, also known as “retaining compound” are used for this application. However the effect of the adhesive nature and of its mechanical and adhesive responses on the performance of the hybrid joint is still unclear. The aim of the present research is to improve the understanding of the behaviour of different adhesives, including rigid epoxies and flexible polyurethanes, in the presence of an interference-fit. Static strength of bonded and unbonded interference fit joints have been compared in order to investigate the role of the different adhesives.     

Adhesive joints between carbon fiber and aluminum foam reinforced by surface‐treated aramid fibers

Short aramid fibers have been successfully used to reinforce the interface adhesive property between carbon fiber/epoxy composites and aluminum foam, and to form aramid‐fiber “composite adhesive joints.” In this study, to further improve the reinforcing effect of the aramid‐fiber‐reinforced adhesive joints, aramid fibers were ultrasonic treated to conduct different surface conditions. Critical energy release rate of the carbon fiber/aluminum foam sandwich beams with as‐received and treated interfacial aramid fibers were measured to study the influence of the surface treatment on aramid fibers. It was found that reinforcements in critical energy release rate were achieved for all samples with treated aramid fiber as measured under double cantilever beam condition. The interfacial characteristics of the short aramid fibers with different surface condition were investigated and discussed based on scanning electron microscopy observations. It is suggested that advanced bonding between aramid fibers and epoxy resin was conducted after surface treatment, and more energy was therefore absorbed through fiber bridging during crack opening and extension process. POLYM. COMPOS., 36:192–197, 2015. © 2014 Society of Plastics Engineers     

Glass bond adhesive strength improvement by DCSBD atmospheric-pressure plasma treatment

A novel type of dielectric barrier discharge, the so called Diffuse Coplanar Surface Barrier Discharge (DCSBD), has been used for the treatment of glass surfaces prior to their bonding with a UV-curing acrylate adhesive. The DCSBD is a source of diffuse, atmospheric-pressure “cold” plasma with power densities reaching ~ 100 W/cm³ even in strong electronegative gases (e.g. pure oxygen) without any admixture of helium or argon. The plasma is generated in a thin, ~ 0.5 mm thick layer suitable for the treatment of flat materials like glass or polymeric foils. The plasma treatment of glass results in an overall decrease in organic contamination of the surface and, possibly, in an increase in the surface density of –OH groups promoting reactions on the glass surface, both effects being beneficial for adhesive properties of the glass surface. Plasma treated adhesive joints have been subjected to mechanical testing according to the ISO 4587 standard. Results indicate more than 45% improvement in the plasma treated joints adhesive strength, when compared to the untreated reference joints. Moreover, the different cleaning protocols used indicate, that it will be possible to substitute distilled water instead of ethanol in the cleaning procedure, which is a rather expensive and hazardous chemical.     

Improvement of the Durability of Zinc-Coated Steel/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electroplated-phosphated (EZ2) steel have been investigated. The degradation mechanisms have been studied after three accelerated ageing tests: the “cataplasme humide” (“C.H.T.”), immersion (“I.T.”), and salt spray (“S.S.T.”) tests. X-ray photoelectron spectroscopy (XPS) analysis of fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that intergranular corrosion phenomena account for poorer performances of the hot-dipped galvanized substrate during “C.H.T.” and “I.T.” Silane coupling agents were successfully used as primers on both substrates to increase the hydrolytic stability of the metal/adhesive interface. XPS results indicate that both the interfacial dissolution of the phosphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear to depend on the nature of the silane coupling agents. Alkylsilanes have been found to perform as well as silanes having a group capable of reacting with the epoxy/dicyandiamide system.

Additional tests were carried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/silane mixture prior to bonding.

When corrosion was the controlling degradation mechanism as is the case during the salt spray test, silane treated specimens did not generally perform better than control specimens.     

Determination of the strain distribution in adhesive joints using Fiber Bragg Grating (FBG)

This work focuses on the development and testing of a technique used to measure strain levels inside an adhesive joint. As more industries adopt high performance structural adhesives, the need for structural monitoring and quality control of adhesive joints rises. The method presented in this work, based on optic fibers, is proposed as a possible means for real-time health monitoring of adhesive connections. In the first part of this work a procedure for embedding optical fibers etched with Bragg sensors is explained. Instrumented, single lap joints were fabricated and subjected to tensile test. The results were compared with finite element models to ensure the accuracy and provide a better understanding of the measurement process.     

Fracture toughness of adhesive joints. II. Temperature and rate dependencies of mode I fracture toughness and adhesive tensile strength

It has been known that adhesive strength shows temperature and rate dependencies reflecting visoelastic properties of an adhesive. Similarly, a critical strain energy release rate is expected to show temperature and time dependencies deformation and fracture of the adhesive occurs at the time of measurement of the critical strain energy release rate, which is a kind of fracture mechanical parameter for adhesive joints. The term “critical strain energy release rate” has usually been called “fracture toughness.” In this study, the critical strain energy release rate (G_IC) of the opening mode was called mode I fracture toughness. G_IC was measured over a wide range of temperatures and rates, and then a master curve was obtained by applying the temperature–rate superposition principle to the obtained data. Also, on the relation between G_IC and adhesive tensile strength is discussed. © 1995 John Wiley & Sons, Inc.     

The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability

The interface in aluminium bonded structures can be revealed by ultramicrotomy and subsequently studied by transmission electron microscopy. By these means, the more usual surface pretreatments encountered, have been characterised in depth.

A similar examination has been effected following exposure of bonded joints (floating roller peel specimens) to 85% relative humidity at 70°C. Although a drop in peel performance is noted over the exposure time, interfacial examination reveals little damage to the adhesive or adherend. Possible mechanisms for bond strength reduction are discussed: subtle undermining of the alumina film and disruption of physico-chemical bonds across the interface. Both are initiated by moisture reaching the alumina film, either passing along the interface itself or travelling through the adhesive matrix. Also considered are the effects of surface pretreatment and “oxide” penetration, by the adhesive, on durability.

The effect of priming the adherend surface prior to bonding, using a heavily strontium chromate filled adhesive primer, is mentioned and its possible influence on durability is briefly discussed.     

Strength of resin-coated-adhesive-bonded double lap-shear pultrusion joints at ambient temperature

The inherited adhesion limitation of polyester and vinyl ester resin-based pultruded GFRP makes pultrusions difficult to bond, especially when a thixotropic adhesive is used. While such an adhesive is necessary for gap filling, it has a limited wettability. Therefore, coating the adherend with low-viscosity epoxy resin, prior to bonding, improves wetting and hence increases joint strength. The paper describes the experimental methodology to achieve this, using double lap-shear (DLS) joints with various materials combinations. A significant strength improvement was reached as a result of coating the inner adherend in conjunction with using a “high adhesion” outer adherend. To further understand the effect of coating, numerical stress analysis was undertaken, including preliminary micro-models representing the composite/adhesive interface as well as overall DLS models.     

An updated review of adhesively bonded joints in composite materials

Continuing interest and more developments in recent years indicated that it would be useful to update Banea and da Silva paper entitled “Adhesively bonded joints in composite materials: an overview”. This paper presents an updated review of adhesively bonded joints in composite materials, which covers articles published from 2009 to 2016. The main parameters that affect the performance of bonded joints such as surface treatment, joint configuration, geometric and material parameters, failure mode etc. are discussed. The environmental factors such as pre-bond moisture, moisture and temperature are also discussed in detail and how they affect the durability of adhesive joints. Lots of shortcomings were resolved during the last years by developing new materials, new methods and models. However, there is still a potential to evaluate and identify the best possible combination of parameters which would give the best performance of composite bonded joints.     

Effect of material on the mechanical behaviour of adhesive joints for the automotive industry

One parameter that influences adhesively bonded joints performance is the adherend material and its effect should be taken into consideration in the design of adhesive joints. In this work, the effect of material on the mechanical behaviour of adhesive joints was investigated experimentally and numerically by single lap joints (SLJs) with different adherend materials (high strength steel, low strength steel and composite). The adhesives selected were two new modern tough structural adhesives used in the automotive industry. It was found that, for relatively short overlaps in SLJs bonded with structural modern tough adhesives, failure is dominated by adhesive global yielding and the influence of material on joint strength is not significant. For larger overlaps, the failure is not anymore due to global yielding and the effect of material becomes more important. Moreover, it was possible to evaluate which adhesive is more suited for each material.     

Review on adhesive joints and their application in hybrid composite structures

Composites have been used extensively in various engineering applications including automotive, aerospace, and building industries. Hybrid composites made from two or more different reinforcements show enhanced mechanical properties required for advanced engineering applications. Several issues in composites were resolved during the last few years through the development of new materials, new methods and models for hybrid joints. Many components in automobile are joined together either by permanent or temporary fastener such as rivets, welding joint and adhesively bonded joints. Increasing use of bonded structures is envisaged for reducing fastener count and riveted joints and there by drastically reducing assembly cost. Adhesive bonding has been applied successfully in many technologies. In this paper, scientific work on adhesively bonded composites and hybrid composites are reviewed and discussed. Several parameters such as surface treatment, joint configuration, material properties, geometric parameters, failure modes, etc. that affect the performance of adhesive bonded joints are discussed. Environmental factors like pre-bond moisture and temperature, method of adhesive application are also cited in detail. A specific case of adhesive joints in hybrid bonded-bolted joints is elaborated. As new applications are expanding in the field of composites joining and adhesive joints, it is imperative to use information on multiple adhesives and their behaviour in different environmental conditions to develop improved adhesive joint structure in mechanical applications.     

Photoelastic Stress Analysis of Bonded Lap Shear Joints Having Thermoplastic Adherends

The effects of substrate stiffness and modulus on joint strength and stress distribution were investigated for a series of nylon substrates bonded with an epoxy adhesive. Substrate stiffness and modulus were controlled by the level of glass filler in the resin. Single lap shear samples having both identical (“self-bonded”) and dissimilar (“cross-bonded”) substrates were investigated. For the self-bonded samples, lap shear strength was found to increase with increasing substrate modulus and stiffness. The strengths of the cross-bonded samples were intermediate to the strengths of the corresponding self-bonded samples. Photoelastic techniques were used to observe stress patterns in the lap joints during testing. One type of stress pattern was observed for all self-bonded samples regardless of substrate stiffness. Two patterns, one for the stiff substrate and one for the more flexible substrate, were observed for cross-bonded samples. The photoelastic analysis agreed qualitatively with predictions of stress distributions based on linear elastic and linear elastic/perfectly plastic theoretical models.     

Analysis of the Influence of Geometric Parameters on the Stress Distributions in Adhesively Bonded Scarf Joints Using 2D Models Under Elastic Assumption

The scarf joint is a usual experimental assembly employed to analyze the mechanical behavior of an adhesive. In fact, using a unique type of bonded assembly with a classic tensile testing machine, various tensile-shear loadings of the adhesive can be applied by changing the value of the scarf angle. In this paper, accurate numerical analyses of the stress distributions within the adhesive in scarf joints under elastic assumption using 2D models are developed. Numerical results underline the influence of the adhesive thickness and mainly the influence of the scarf angle on the edge effects, and confirm the presence of an optimal scarf angle associated with very low stress concentrations. Moreover, the use of a suited elastic limit for the adhesive, defined from the two stress invariants, hydrostatic stress and von Mises equivalent stress, allows the more stressed parts of the adhesive with respect to the scarf angle to be defined. These results also underline the possible influences of the edge effects on the experimental results, i.e., possible crack initiations close to the free edges of the adhesive for some scarf angles. Finally, it is shown that a little modification of the free edges of the adhesive (a so-called “cleaning”) can strongly reduce the influences of the edge effects and thus can improve the experimental results for a wide range of scarf angles.     

Ultrasonic Determination of the Cohesive Properties of Bonded Joints by Measurement of Reflection Coefficient and Bondline Transit Time

An ultrasonic nondestructive technique for the quantitative determination of the cohesive properties of adhesive joints based on the measurement of the reflection coefficient from the top adhesive/adherend interface and the bondline transit time has been developed. The method requires access to only one side of the joint and, for joints with typical aerospace geometries, it can be implemented using a single transducer with a centre frequency below 50 MHz. The technique has been used to determine the longitudinal bulk wave velocity in aluminium-epoxy-aluminium joints to within ±6% of the nominal values determined from bulk samples. The bondline thickness of the samples tested was evaluated to within micrometer accuracy, and thickness variations within the scan area were detected to much better than micrometer accuracy. The method has been tested successfully on joints made with two-part epoxies and with film adhesives containing a “scrim” carrier, and it has also been shown that the different standard adherend preparation procedures have a negligible effect on the results. The method, therefore, promises to provide a reliable, nondestructive means of measuring the cohesive properties of a bonded joint and represents a significant advance on the currently available technology.     

Comments on “Fatigue Performance of Two Structural Adhesives” [J. Adhesion 26, 273-291 (1988)]

Since we at Imperial College also have an extensive research programme concerned with the fatigue performance of structural adhesives and adhesive joints, I read with interest the paper' on the “Fatigue Performance of Two Structural Adhesives”.     

Debonding on command of multi-material adhesive joints

ABSTRACT

In this work an innovative technique for multi-material adhesive joints debonding by combining the inductive heating method and the use of thermally expandable particles (TEPs) is presented. First, single lap joints (SLJs) using various combinations of adherends (high-strength steel (HS), aluminum (Al), and carbon fiber reinforced plastics (CFRP)) were fabricated and tested to assess the influence of TEP content on the lap-shear strength of the joints. Further, the ability of the TEP-modified joints to support temperature-controlled debonding was evaluated. It was shown that the control of debonding process by temperature is possible. The temperature needed for debonding is a function of TEP content and can be lowered by increasing the TEP content. Relatively similar debonding temperatures were found for multi-material/dissimilar joints debonding compared with similar joints, but more induction heating power is generally necessary to disassemble multi-material adhesive joints.